1. Field of the Invention
The present invention relates to a direct access storage device (DASD) of the type in which a read/write transducer head is moved above a storage media for reading and writing data, and more particularly to improved servo writing methods and apparatus for a direct access storage device.
2. Description of the Prior Art
Disk drive units incorporating stacked, commonly rotated rigid magnetic disks are used for storage of data in magnetic form on the disk surfaces. Transducer heads driven in a path toward and away from the drive axis write data to the disks and read data from the disks. Data is recorded in concentric data information tracks arrayed on the surfaces of the disks.
All DASD units must have a method to position each data head over the proper radial location to write a track and again, to position it very close to the same location to read the track. With the higher level files using a voice coil type of actuator, a feedback mechanism must be provided to locate and stably hold the head on a given track. Typically, track accessing and track following is provided by a magnetically written pattern in the DASD unit. A dedicated servo system employs one surface of one of the disks in the DASD on which to have all the tracking and access information. A sector servo system uses small portions of tracks between each or between several sectors on each track of each data surface to provide the tracking and access information. A hybrid servo system uses both to obtain advantages of each type of servo.
Typically the servo patterns are written on the disk or disks with a special servo writer system. This system usually includes a laser measured access system to accurately position the heads. This requires a retro-reflector to be attached to the file actuator. The system also includes a clock head or heads to write timing information. Each file that is to be servo written must be firmly clamped to the servo writer to maintain accurate positioning between the two machines. The servo writer must be used in a clean area since the file must be open during the track writing process. This increases the cost and increases the probability of contaminating the file.
In clamping the DASD unit to the servo writer, the natural resonances of the file are significantly changed. Thus while tracks written on the servo writer appear to be nearly perfect, they change when the file is removed and the resonances change, the servo system does not follow the servo tracks perfectly, creating repeatable runout, which makes the determination of being on track difficult.
Examples of other known servo systems are provided by U.S. Pat. Nos. Oliver et al., 4,414,589; Penniman, 4,530,019; and Berger, 4,531,167.
Oliver et al., U.S. Pat. No. 4,414,589 discloses an embedded servo track following system and method for writing servo tracks that does not require a servo writer to write embedded servo data. However, a clock track for very accurate timing is required for providing an index of disc position and a measure of disc speed for writing servo data. The clock track is written by writing a single pulse on a fixed magnetic clock head, phase-lock looping to an intermediate clock track, which is written on a moving head, and then phase-lock looping up to the final clock track which is written on the fixed clock track head.
Berger, U.S. Pat. No. 4,531,167 discloses a portable servo writer system for writing clock and servo tracks either on a dedicated disk surface or embedded servo tracks on disks of a magnetic disk drive. Servo tracks are written by providing a master disk cartridge having one fixed head and a dedicated servo surface. The master disk cartridge is installed on the drive and the servo surface of the drive and the embedded servos are written using prewritten clock and index information from the fixed head of the master cartridge.
Penniman, U.S. Pat. No. 4,530,019 discloses a servo pattern including an erased gap followed by an automatic gain control (AGC) information burst followed by a first burst of servo control information followed by a second burst of servo control information. The pattern is written on a disk by the disk drive unit using a mechanical index on an armature associated with the rotation of the disk as a primary time reference, with all other time references being based on a transition between the erased gap and the AGC burst. While the disclosed servo pattern eliminates the need for a separate clock track, the mechanical index, sensors for the mechanical index, and a positioning system are required for moving the read/write head to write the servo pattern.
While the prior art servo writing devices provide improvements, it is desirable to provide an improved method for writing servo information by the disk file and a self servo writing file where a special servo writer system and prewritten patterns on the disks are not required and further that does not require either a clock track timing reference or substantial different circuitry from the normally available circuitry of the disk file.